In recent years, an optical fiber telecom industry utilizes wavelength division multiplexing (WDM) technology to deal with an increase in communication data amount. A wavelength selective switch (WSS) has been known as an optical device constructing a large-capacity optical communication system using the WDM technology.
The WSS is a device which selects any wavelength from a WDM signal input thereto and can allocate the selected wavelength to any output. Basically, the WSS typically has an input/output portion through which the WDM signal is input or output, a dispersive element which demultiplexes and multiplexes the WDM signal, a light-condensing element which condenses a plurality of light beams generated by demultiplexing the signal, and an optical deflection device having optical deflection elements deflecting the light beams to switch output.
As the WSS, a MEMS (Micro Electro Mechanical System) advantageous in terms of optical signal transmission band characteristics, optical loss, and polarization dependency has been mainly used, and a wavelength selective switch having a MEMS mirror has been known as an optical deflection device (for example, see Patent Literature 1).
Meanwhile, such a wavelength selective switch has a challenge in preventing stray light from an unused MEMS mirror from being coupled to a fiber. In order to overcome the challenge, means for shielding light with the use of a drive circuit driving the unused MEMS mirror has been considered; however, since an extra drive circuit is needed to drive the MEMS mirror not normally used, there has been a problem that cost is increased. Thus, there has been adopted a configuration in which while light is partially allowed to pass through an opening formed in a surface of a glass member near the MEMS mirror, a light shielding plate (mask) used to shield other portions from light is attached with an adhesive to thereby suppress stray light.
FIG. 10 is a schematic view showing a conventional wavelength selective switch module 100, and the module 100 includes a light-transmittable transparent plate 2, a light shielding plate 3 which is adhered to the transparent plate 2 with an adhesive 4, has an opening 31 through which light can pass, and shields incident light other than light entering the opening 31, MEMS mirrors 6 which are located on the side opposite to the light shielding plate 3 across the transparent plate 2 and receive incident light, and a substrate 7 with the MEMS mirrors 6 mounted thereon. In FIG. 10, in the MEMS mirrors 6, a region receiving incident light is described as a “used mirror region”, and a region shielded from light by the light shielding plate 3 and thus receiving no incident light is described as an “unused mirror region”.